1) Both are trying their hardest to 'save' humanity
2) Both are somehow removed from human hands and implemented into Alien technology
3) Both go rampant

In the ending of Halo 2: Durandal, the ending states:

While Tau Ceti was being nuked down to bedrock in 2794, Pfhor scientists disassembled and removed the AI Leela from the Marathon, loading her aboard a vessel bound for the Pfhor homeworld. But the ship fell into the hands of a Nar privateer between jumps at Beta Naxos, and was never seen by the Pfhor again.

Thinking the cargo little more than scrap, the Nar captain sold the Pfhor ship, Leela and all, to a Vylae merchant. The subsequent crash of the Vylae FTL network when Leela was reassembled and reactivated is still legendary in the annals of rampancy, and the Vylae have long since accepted that they will never expunge her from their fifteen-world network.

While we can assume that Cortana and Leela are two seperate AI's, I cannot fail but see many similarities in their decisions. This leads me to believe that perhaps Cortana is a copy or a clone of Leela. I was also under the impression that they are actually the same entity, but I disregarded that after playing Marathon Infinity.

Just a thought.

I'd like to see the two of them sit down at a table and talk for a while. They'd share a lot of common ground.

As I was skimming through Wikipedia about Marathon (Just downloaded it), I came across the W'rkncacnter.

A piece of terminal text from M:Durandal said this:"In primordial space, timeless creatures made waves. These waves created us and the others. Waves were the battles, and the battles were waves. "Fleeing all W'rkncacnter, Yrro and Pthia settled upon Lh'owon." Pthia & Yrro; Librarian & Didact. Librarian stayed on Earth, away from the Flood. The Didact lived in the Ark, also away from the Flood (debatable)."They brought the S'pht, servants who began to shape the deserts of Lh'owon into marsh and sea, rivers and forests."
The various constructs they made, such as Sentinels & Constructors. Lh'owon could be a Halo, or a terraformed planet, or something else. "They made sisters for Lh'owon to protect and maintain the paradise." The other Halos and Installations."When the W'rkncacnter came, Pthia was killed, and Yrro in anger, flung the W'rkncacnter into the sun. The sun burned them, but they swam on its surface." The activation of the Halos.
Remember that the Halos worked by starving The Infection. So, in a way(by killing the sentient Gravemind, or the starvation itself), the Sun(Halos) burned them, but they swam on its surface (The Flood survived the original pulse, but died from Starvation).

Also, it said in Infinity that a W'rkncacnter was imprisoned on the Sun. Like the Flood in Installation 04 (and 05).
The Pfhor used a trih xeem device to send the star into early nova, and the creature was released, to the horror and destruction of the Pfhor.
The Covenant tricked(or simply didn't know about the Flood) the humans into opening the Chamber which contained the Flood, and most of the Covies in Installation 05 perished when the Parasite was released, to their horror.

In the Halo Graphic Novel story The Last Voyage of the Infinite Succor an Elite of Commander rank boards the Infinite Succor as part of a team sent to recapture the ship after a, presumed to be human, force attacks it. The Commander encounters a Prophet (The Minister of Etiology) who has been observing the attackers and informs the Elite Commander it is a Flood infection (the Commander was previously unaware of what his team were being attacked by, even after establishing it wasn't humans) and that they were "building" a "creature" that he believed "serves as their leader and organizer" (a Gravemind). While escaping the ship the Commander is attacked by an infected Elite armed with two Energy Swords, a duel commences and on the right-hand side of panel four on page fifty-three the Commander has half of his jaw cut off by the Flood Elite. The half-jawed Commander leaves the Infinite Succor as a two minutes to slipspace jump warning is sounded in the second last panel and the story ends with the Commander's Phantom leaving the Inifinite Succor which is intact in the background. The Commander had previously explained to the Prophet it would take fifteen minutes to arrange the Covenant fleet to fire on the Infinite Succor, so it's entirely possible the ship escaped. Though the "Last Voyage" element of the title suggests otherwise*. The story takes place while the Chief is on his way to find the missing Captain Keyes and his team of Marines.

So not only have the Covenant enountered Graveminds in the past, but Half-Jaw himself saw one in the earlier stages of construction and barely escaped. I'd imagine someone would have brought this up in the forums nearer the time but I thought I'd make sure.

*You can elaborate on this by saying that once the ship was completely taken under Flood control it ceased to be the Covenant ship Infinite Succor, in which case the title would fit should the ship have escaped.

Darren.

Seems that slipped off our minds. We need some logic from time to time. ;)